Observation of Auroral-likePeaked Electron Distributions

at Mars

D.A. Brain, J.S. Halekas,

M.O. Fillingim, R.J. Lillis, L.M. Peticolas,

R.P. Lin, J.G. Luhmann, D.L. Mitchell, G.T. Delory,

S.W. Bougher, M.H. Acuna, H. Reme

UC Berkeley Space Sciences Lab

brain@ssl.berkeley.edu

SA53B-1166

30-second Summary

• Auroral emission was recently observed at Mars!

• We observe electrons responsible for auroral emission

• We see ~13,000 peaked electron energy spectra(very similar to terrestrial auroral electrons)

• Spectra associated with open and closed crustal field lines

• A few spectra can produce previously observed UV emission

• Most energetic spectra may be associated with SEP events

• Controlled by Mars orientation, IMF direction, SW pressure

Aurora Discovered at Mars!

Bertaux et al., Nature, 2005

• Mars Express SPICAM (UV) instrument

• Observed at 129 km, ~120°SZA, ~19:00 LT

• Wavelengths associated with CO, CO2+, O

• Associated with likely magnetic cusp

• Intensity difficult to explain

• Only one reported emission event

Is Martian auroral emission commonplace?

Martian Crustal Fields|B| Elevation Angle

Radial Field

Topology

‘Up’‘Down’

ClosedOpen

Draped

Martian Field Topology

“Closed” field lines “Open” field lines

Electrons carry information about the topology of magnetic field lines observed at S/C altitudes

Tools: MGS MAG/ER DataMGS Orbit

circular, 400 km, polar, 2am6 years of data

ER Electron Angular Distributions

used to identify cusps/closed field lines Angular resolution = 14° 22.5° data every 2-8 s

IMF Direction and SW Pressure

Determined indirectly from dayside MAG data, once per orbit

ER Electron Energy Spectra

used to identify plasma population 10 eV - 20 keV, E/E ~ 25%, 12 - 48 s, FOV = 14° 360°

Auroral Electron Spectra

Earth Mars

FAST MGS

Data courtesy C. Carlson

A Martian Auroral Electron Event

A B C E

Auroral-like peaked electron energy spectra

D

currents

A Martian Auroral Electron Event

A B C D ESN

A, E - Open field lines (‘cusps’)

B, D - Closed field lines, contain trapped electrons

C - Closed field lines, devoid of electrons

Auroral electrons observed

MGS orbit

Acceleration Mechanism

v

v

v

v

v

v

Terrestrial aurorae typically associated with charged particles accelerated by a potential.

Are the isotropic Martian electron distributions consistent with this paradigm?

1. Source

Electrons initially isotropic

Te(sheath) ≈ 40 eV

2. Acceleration

Potential boosts v

V ≈ 200 eV (sample event) ≈ 24 electrons field-aligned

3. Isotropization

As B increases, v v

Bobserved / Bsource ≈ 6 source could be sheath;waves may also contribute

€

sin2α

Bsource=

sin2 90o

Bobserved

Yes, if source is high temperature region and potential drop large, or waves contribute

Another Event

BeamedEvidence for waves, field rotation

Secondary eletrons from atmosphere?

Still Another Event

Closed field lines?

Global Distribution

- MEX observation - MGS event

~13,000 identified spectra fluxes 10-10000 higher than typical nightside spectra

Emission and Energy Deposition• Emission estimated using electron transport model

Peaked Spectrum Tail Spectrum

Downward energy flux

ergs/cm2/s

5.810-3 0.510-3

Column emission

(289.7 nm)

4 R 0.3 R

Column emission

(1304 nm)

1.5 R 0.3 R

Column emission

(1356 nm)

0.6 R 0.1 R

Peak emission altitude 145 km 151 km

• Mars Express: 71±42R at 129 km for 289.7 nm

• Our peaked spectrum 17 too small

• Options:Require more intense spectrumNeed to integrate horizontally

Emission and Energy Deposition

Some auroral-like spectra 17 more intense than our example

MGS has observed electron distributions capable of producing the emission intensities observed by Mars Express

Example spectrum

Distribution of downward energy flux of peaked electron spectra Energy flux organized by location

SEP Event during MEX Observation

ER instrument background countrate

SW Pressure proxy MEX Event

Did the SEP event contribute to the MEX observation? Are SEP events prerequisites for intense auroral emission?

10-20 keV electrons

EnergyC

oun

t R

ate

ER

10’s MeV ions

Energy

Cou

nt

Rat

e

SEP Correlation at MGS?Background Count Rate

~50% of most energetic peaked spectra occur

during SEP events

External ControlSeason IMF Direction Upstream Pressure

120-180E, 60-90S everywhere else

duskward

dawnward

summer

winter high P

low P

Future Directions• Better identify and characterize peaked electron spectra

• Correlate intensity with external conditions

• Organize by cusp location/polarity

• Investigate correlation with waves, field rotations

• Determine role of SEPs in creating emission

• Add converging magnetic fields to emission calculations( see Poster #SA51B-1142 by M. Fillingim et al. - today! )

• Compare to Mars Express ASPERA-3 and SPICAM observations

Goals: Identify source regionIdentify source mechanism and role of reconnectionFully characterize resulting emissionCompare to other planetary aurorae